Re: [PATCH v2] drm/panic: allow overriding panic QR code URL

[Nahida Buer]

Thanks for the review.

My motivation was slightly different from disabling the URL when the
configured endpoint is untrusted or unavailable. I wanted to keep the
URL QR mode while allowing the same kernel image to use a different
reporting endpoint depending on the deployment.

It also allows enabling the compressed URL mode without rebuilding the
kernel. With an empty CONFIG_DRM_PANIC_SCREEN_QR_CODE_URL, the QR code
contains the raw kmsg. Supplying drm.panic_qr_url= at boot instead
switches it to the URL format, where the kmsg is compressed with zlib.

This could be useful for a distribution or a locally built kernel image
used on multiple systems: the image can keep a neutral/default
configuration, while individual systems can opt into a specific panic
report collector and compressed QR payload at boot time.

panic_screen=user or panic_screen=kmsg only disables the URL QR mode;
it does not provide a way to keep that mode while changing or enabling
the reporting endpoint.

The parameter is read-only after initialization, so it only selects the
endpoint at boot/module load time.

If keeping the URL fixed at build time is intentional policy for DRM
panic, though, I'm fine with dropping the patch.

Jocelyn Falempe <jfalempe@xxxxxxxxxx> 于2026年10月3日周六 06:56写道：
>
> On 10/3/26 00:20, 纳西妲 · Nahida wrote:
> > The base URL used for the panic QR code is currently fixed at build time
> > by CONFIG_DRM_PANIC_SCREEN_QR_CODE_URL. Changing the panic report
> > endpoint therefore requires rebuilding the kernel.
> >
> > Add a drm.panic_qr_url module parameter to allow overriding the URL at
> > boot or module load time, while keeping the Kconfig value as the
> > default.
> >
> > Keep the parameter read-only after initialization so the panic path can
> > access it without additional synchronization.
> >
> > Expose the configured URL to the DRM panic helper through an internal
> > getter so the helper can use the runtime value when generating the QR
> > code.
>
> The code looks good, but I think it shouldn't be allowed to change this
> setting at boot time.
> You can already set the drm_panic screen to "user" or "kmsg" in the
> kernel command line if you don't trust the default URL, or if the URL is
> no more reachable.
> It introduces some complexity, and I don't really see the benefit.
>
> Best regards,
>
> --
>
> Jocelyn>
> > Assisted-by: LLM
> > Signed-off-by: 纳西妲 · Nahida <me@xxxxxxxxx>
> > ---
> > v2:
> > - Grow the formatted URL buffer to accommodate the maximum 255-byte
> >    runtime base URL.
> > - Reject unexpectedly truncated formatted URLs.
> > - Test the maximum-length runtime URL.
> >
> > v1: https://lore.kernel.org/r/20261002203140.121165-1-me@xxxxxxxxx
> >
> > Tested on x86_64:
> > - Full modular build with DRM=m and DRM_KMS_HELPER=m.
> > - QEMU/virtio-gpu runtime override with a non-empty Kconfig default URL.
> > - QEMU/virtio-gpu runtime fallback to the Kconfig default URL.
> > - QEMU/virtio-gpu runtime override with an empty Kconfig default URL.
> > - QEMU/virtio-gpu raw kmsg QR fallback with no configured URL.
> > - QEMU/virtio-gpu runtime override with a 255-byte base URL.
> >
> >   drivers/gpu/drm/Kconfig              |  9 +++++----
> >   drivers/gpu/drm/drm_panic.c          |  9 +++++++++
> >   drivers/gpu/drm/drm_panic_helper.c   | 17 +++++++++++------
> >   drivers/gpu/drm/drm_panic_internal.h |  4 ++++
> >   4 files changed, 29 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> > index 61111d50b..3d9d421f0 100644
> > --- a/drivers/gpu/drm/Kconfig
> > +++ b/drivers/gpu/drm/Kconfig
> > @@ -111,10 +111,11 @@ config DRM_PANIC_SCREEN_QR_CODE_URL
> >       string "Base URL of the QR code in the panic screen"
> >       depends on DRM_PANIC_SCREEN_QR_CODE
> >       help
> > -       This option sets the base URL to report the kernel panic. If it's set
> > -       the QR code will contain the URL and the kmsg compressed with zlib as
> > -       a URL parameter. If it's empty, the QR code will contain the kmsg as
> > -       uncompressed text only.
> > +       This option sets the default base URL to report the kernel panic. If
> > +       it's set the QR code will contain the URL and the kmsg compressed with
> > +       zlib as a URL parameter. If it's empty, the QR code will contain the
> > +       kmsg as uncompressed text only. It can be overridden with the
> > +       drm.panic_qr_url=<url> kernel parameter.
> >         There is a demo code in javascript, to decode and uncompress the kmsg
> >         data from the URL parameter at https://github.com/kdj0c/panic_report
> >
> > diff --git a/drivers/gpu/drm/drm_panic.c b/drivers/gpu/drm/drm_panic.c
> > index 9fe97c4ed..463544f71 100644
> > --- a/drivers/gpu/drm/drm_panic.c
> > +++ b/drivers/gpu/drm/drm_panic.c
> > @@ -49,6 +49,15 @@ MODULE_LICENSE("GPL");
> >   static uint panic_qr_version = CONFIG_DRM_PANIC_SCREEN_QR_VERSION;
> >   module_param(panic_qr_version, uint, 0644);
> >   MODULE_PARM_DESC(panic_qr_version, "maximum version (size) of the QR code");
> > +static char panic_qr_url[256] = CONFIG_DRM_PANIC_SCREEN_QR_CODE_URL;
> > +module_param_string(panic_qr_url, panic_qr_url, sizeof(panic_qr_url), 0444);
> > +MODULE_PARM_DESC(panic_qr_url, "base URL of the QR code in the panic screen");
> > +
> > +const char *drm_panic_get_qr_url(void)
> > +{
> > +     return panic_qr_url;
> > +}
> > +EXPORT_SYMBOL_GPL(drm_panic_get_qr_url);
> >   #endif
> >
> >   static enum drm_panic_type drm_panic_type = -1;
> > diff --git a/drivers/gpu/drm/drm_panic_helper.c b/drivers/gpu/drm/drm_panic_helper.c
> > index 8913c3d22..b0ea0b4e8 100644
> > --- a/drivers/gpu/drm/drm_panic_helper.c
> > +++ b/drivers/gpu/drm/drm_panic_helper.c
> > @@ -609,17 +609,20 @@ static void drm_panic_helper_qr_exit(void)
> >       stream.workspace = NULL;
> >   }
> >
> > -static int drm_panic_helper_get_qr_code_url(u8 **qr_image, unsigned int qr_version)
> > +static int drm_panic_helper_get_qr_code_url(u8 **qr_image, unsigned int qr_version,
> > +                                         const char *qr_url)
> >   {
> >       struct kmsg_dump_iter iter;
> > -     char url[256];
> > +     char url[512];
> >       size_t kmsg_len, max_kmsg_size;
> >       char *kmsg;
> >       int max_qr_data_size, url_len;
> >
> >       url_len = snprintf(url, sizeof(url), "%s?a=%s&v=%s&z=",
> > -                        CONFIG_DRM_PANIC_SCREEN_QR_CODE_URL,
> > -                        utsname()->machine, utsname()->release);
> > +                        qr_url, utsname()->machine, utsname()->release);
> > +
> > +     if (url_len >= (int)sizeof(url))
> > +             return -E2BIG;
> >
> >       max_qr_data_size = drm_panic_helper_qr_max_data_size(qr_version, url_len);
> >       max_kmsg_size = min(MAX_ZLIB_RATIO * max_qr_data_size, QR_BUFFER1_SIZE);
> > @@ -686,8 +689,10 @@ static int drm_panic_helper_get_qr_code_raw(u8 **qr_image, unsigned int qr_versi
> >
> >   static int drm_panic_helper_get_qr_code(u8 **qr_image, unsigned int qr_version)
> >   {
> > -     if (strlen(CONFIG_DRM_PANIC_SCREEN_QR_CODE_URL) > 0)
> > -             return drm_panic_helper_get_qr_code_url(qr_image, qr_version);
> > +     const char *qr_url = drm_panic_get_qr_url();
> > +
> > +     if (qr_url[0])
> > +             return drm_panic_helper_get_qr_code_url(qr_image, qr_version, qr_url);
> >       else
> >               return drm_panic_helper_get_qr_code_raw(qr_image, qr_version);
> >   }
> > diff --git a/drivers/gpu/drm/drm_panic_internal.h b/drivers/gpu/drm/drm_panic_internal.h
> > index fe6d514a0..f33552265 100644
> > --- a/drivers/gpu/drm/drm_panic_internal.h
> > +++ b/drivers/gpu/drm/drm_panic_internal.h
> > @@ -14,6 +14,10 @@ struct drm_device;
> >
> >   #ifdef CONFIG_DRM_PANIC
> >
> > +#if IS_ENABLED(CONFIG_DRM_PANIC_SCREEN_QR_CODE)
> > +const char *drm_panic_get_qr_url(void);
> > +#endif
> > +
> >   /**
> >    * drm_panic_lock - protect panic printing relevant state
> >    * @dev: struct drm_device
> >
> > base-commit: 54e61ea6c492d69a1717018826d8340810c812eb
>